A vehicle that has become stuck in a depression-like hollow with spinning wheels, so that the vehicle cannot be driven directly out of the depression because the subsoil friction is insufficient or the subsoil is too yielding, can often be freed from that situation by a so-termed rocking-free maneuver. One possibility for emerging from the depression without outside help consists in rocking the vehicle by driving it cyclically forward and in reverse. In physical terms the forward and backward driving or forward and backward rolling vehicle constitutes an oscillating system which can be energized by a suitable input of energy until the kinetic energy suffices to overcome the edge of the depression.
In vehicles with a manual transmission the driver can carry out a rocking-free process of that kind by rhythmic actuation of the starting clutch and the accelerator pedal and by repeatedly changing between forward and reverse driving, in order successively to rock the vehicle free.
In vehicles with an automated or automatic transmission automatic rocking-free modes are known, which the driver can switch into or which are automatically engaged by virtue of an automatic rocking-free recognition function. The rocking-free process itself then takes place automatically such that a periodically increasing and decreasing drive torque in the forward and/or the reverse direction is produced. The drive torque can for example be produced by a combustion engine, while a gear of the transmission is automatically engaged or disengaged or while a shift automatically takes place repeatedly between a forward gear and a reverse gear. In the case of a hybrid or electric drive, the drive direction can be changed almost without any delay by reversing the current flow in an electric drive motor.
DE 10 2010 043 250 A1 describes a method for the automatic rocking-free of a motor vehicle having an automatic or automated transmission, in which rotational direction information for detecting the travel direction of the motor vehicle at the time is used. A rocking-free function is started if a rocking-free situation is recognized automatically for the motor vehicle or if the driver specifies the same by actuating an operating element, and the function is automatically terminated when the motor vehicle reaches a travel-path dependent or a time-dependent limit. Starting from a position with a stationary wheel in a depression from which the motor vehicle has to be moved clear, to rock it free a drive torque is first built up which acts in a desired travel direction as determined by the gear engaged until the motor vehicle moving in the desired travel direction reaches a forward travel direction reversal point of the depression. This travel direction reversal point is recognized in that the speed at a non-driven wheel falls to zero or the movement direction of the motor vehicle is reversed. The drive torque is then reduced until the motor vehicle, now moving in a direction opposite to the desired travel direction, reaches a rearward travel direction reversal point, and is then built up again. The process is repeated cyclically, whereby the locations of the reversal points are continually adapted and successively migrate out of the depression until the motor vehicle reaches a forward or rear limit point which marks the end of the depression. The limit points are defined by an admissible travel path from the starting position in the relevant travel direction or by an admissible time during which the drive torque is applied, and are continually adapted.
During rocking free, in addition to the alternate build-up and reduction of the drive torque, when the drive torque is reduced the transmission can be shifted to neutral so that in the neutral transmission position the motor vehicle rolls passively backward, or a shift between a forward and a reverse gear can take place so that an active drive torque acts in both rocking directions.
If an electric motor drive is available, then to change between the drive torque acting in the desired travel direction and the drive torque acting in the opposite direction there is no need for a gear change, since the torque direction can be changed by means of the electric drive.
The rocking free as such is carried out fully automatically. However, the driver can have the option to influence the rocking-free function actively by actuating the accelerator pedal, since the function is activated or deactivated depending on a minimum actuation, i.e. a minimum deflection angle of the accelerator pedal and/or since, depending on the acceleration pedal actuation, in each case a maximum drive torque is determined.
DE 10 2005 023 246 A1 describes a method for operating a motor vehicle, in which for a rocking-free function the information from rotational direction sensors is used in order to detect the travel direction of the motor vehicle at the time. When a motor vehicle driving actively toward an edge of a depression in a desired direction, i.e. with a gear engaged and powered by a drive engine, comes to rest, a motor vehicle clutch arranged in the torque flow is opened. When the motor vehicle approaches an opposite depression edge passively in the opposite direction, i.e. when it is rolling back under no power, then when the vehicle comes to rest the motor vehicle clutch is closed again for a new active approach in the desired direction. Alternatively, for the purpose of rocking free, when the motor vehicle comes to rest after driving forward actively a shift is carried out automatically to a reverse gear and correspondingly, after actively driving in reverse, when the motor vehicle comes to rest a shift is carried out to a forward gear.
DE 10 2009 036 058 A1 describes a method for rocking a motor vehicle free, in which a dual-clutch transmission has a rocking-free mode such that in a first partial transmission a forward gear and in a second partial transmission a reverse gear are engaged at the same time, and the drive input is automatically switched between the partial transmissions. The rocking-free mode is automatically activated if, within a short time, the driver shifts the transmission repeatedly between forward drive, reverse and neutral. Or else the driver engages the rocking-free mode manually if he recognizes the need for this or when a display recommends this to him. The vehicle is only operated in the rocking-free mode when the driver actuates the accelerator pedal. Furthermore, a distance sensor system ensures that the rocking-free mode is automatically terminated when a collision with an obstacle is possible if rocking-free continues.
In the case of a vehicle with an automated transmission or with an automatic transmission, if the driver does not want fully automatic rocking free, or if no such fully automatic function is available, or if an existing rocking-free function is to be influenced actively by the driver, then a rocking-free function can be carried out and its sequence controlled by alternate actuation and release of the accelerator pedal. A repeated change of the position of a selector lever between its positions for forward driving, reversing and neutral is also possible. For the driver, however, it is difficult to act in phase during rocking free, i.e. to recognize exactly the respective changeover point between forward and reverse movement and the accordingly most favorable time-point for actuating and releasing the accelerator pedal. For example in a commercial vehicle the driver's cabin can be largely decoupled from a chassis in relation to oscillations by a special cabin suspension system, so that when motor vehicle movement is beginning or coming to an end close to a travel direction reversal point, neither the travel direction nor the movement as such is perceived directly by the driver.